The present invention relates to a sewing machine having a plurality of sewing needles.
For embroidery stitching with various colors, a plurality of colored threads are used in a sewing machine having multiple needles. In a conventional sewing machine having multiple needles, a plurality of needle bars are vertically movably held by a needle bar case. After moving the needle bar case to move a selected one of the needle bar to a predetermined stitching position, a driving power is transmitted to the selected needle bar from a drive source so that the sewing needle is vertically reciprocatingly driven. By co-operation of the sewing needle and a rotary hook, stitching is performed with using a specified colored thread supplied from a thread spool corresponding to the driving sewing needle.
In such a conventional sewing machine having multiple needles, stitching operation speed, judgment timing for judging occurrence in thread breakage, thread cutting timing by a thread cutting mechanism, the relationship between displacement of the sewing needle and a rotation angle of the rotary hook, the relationship between the displacement of the sewing needle and a timing for feeding a workpiece fabric, and a feeding amount of the workpiece fabric are not adjustable. In another conventional sewing machine having multiple needles, these may be adjustable. However, the adjustment is imparted uniformly on all sewing needles.
However, the plurality of needle threads those used in the sewing machine having multiple needles have characteristic different from one another dependent on materials, diameters, and stranding manner, and therefore, tensile strength, stretching and shrinking characteristic and surface smoothness are different from each of the needle threads. Accordingly, even if adjustment is given so as to provide optimum sewing condition with respect to a specific one of the needle threads, this adjustment is also imparted on the other sewing needle, and therefore, this adjustment is not proper to the needle threads associated with the other sewing needles.
To be more specific, provided that operation speed of the sewing needles are evenly adjusted on the premise of a needle thread having an average characteristic, thread cutting may frequently occur if a needle thread having an extremely low tensile strength is also used in one of the sewing needles. To avoid this, if the operation speed of the sewing needles is evenly adjusted with respect to the all sewing needles on the premise of a needle thread having the lowest tensile strength, entire sewing needle are operated at lower speed. This lowers sewing efficiency.
Further, in this type of sewing machine having multiple needles, a thread cutting sensor is provided for making judgment of the cutting or breakage of the needle thread. The thread cutting sensor is provided by a rotation member rotated in frictional contact with the needle thread paying out from the thread spool and a detector such as a photosensor for detecting the rotation state of the rotation member. With this arrangement, occurrence in thread cutting is detected if the rotation member is not rotated in spite of a predetermined numbers of vertically reciprocating movement of the sewing needle. In this case, even if adjustment is made so as to detect the cutting of the thread at a proper timing with respect to a needle thread having an average characteristic, the rotation member may not be rotated in spite of the paying out of the needle thread due to the slippage of the needle thread over the rotation member if the needle thread has a relatively low friction coefficient. As a result, judgment may erroneously fall that the thread breakage occurs in spite of no occurrence of thread cutting.
Reversely, judgment timing for judging the occurrence in thread cutting can be delayed taking such needle thread having the relatively low friction coefficient into consideration. However, judgment timing as to the whole sewing needles are evenly delayed. Accordingly, occurrence in thread cutting cannot be promptly judged with respect to the needle thread having an average characteristic.
Further, in the conventional sewing machine having multiple needles, the needle thread and the bobbin thread are cut by a thread cutting mechanism provided in the machine when a consequential sewing operation is completed. By adjusting operation timing of the thread cutting mechanism, a remaining or residual length of the thread extending from a needle hole through which the thread passes to a cut end of the thread can be changed. Even if the operation timing is adjusted so as to provide an optimum remaining length with respect to the thread having an average characteristic, the desired remaining length cannot be obtained with respect to a thread having a shrinking nature. That is, such largely shrinkable thread shrinks immediately after the thread cutting operation. In some cases, the largely shrinkable thread may be disengaged from the thread hole due to its excessive shrinkage.
Reversely, the thread cutting mechanism can be operated at a proper timing so as to obtain sufficient remaining length of the thread taking the largely shrinkable thread into consideration. However, this adjustment is effected to all sewing needles, and therefore, excessively large remaining length may be provided in case of the thread having an average shrinking characteristic. Such a large remaining length portion of the thread may be involved in a stitch seam, or entangled with a movable component.
Further in the conventional sewing machine having multiple needles, rotation phase of a rotary hook (loop taker) is adjusted in such a manner that a loop seizing beak of the rotary hook reaches a thread trapping position at a needle-rotary hook meet timing when the needle is slightly moved upwardly after the needle is moved to its lower dead center. In this case, a sufficient size of thread loop can be formed and the thread can be trapped by the loop seizing beak at the adjusted rotational phase with respect to the thread having an average characteristic. However, if a largely distortable thread is also used, a resultant loop may be distorted when the loop is growing large. With such a distorted shape of the loop, the thread is not easily trapped by the loop seizing beak.
In order to avoid this problem, a rotational phase adjustment of the rotary hook can be made so as to trap the thread while the loop is not grown so large and the large distortion of the loop has not occurred taking the largely distortable thread into consideration. However, since this adjustment is effected on all sewing needles, rotation phase of the all rotary hooks is also evenly changed. Accordingly, loop trapping may become difficult in accordance with the smaller size in the loop in case of the thread having an average distortional characteristic.
Further, conventionally, the feed of the workpiece fabric is adjusted with respect to the all sewing needles regardless of the feeding amount or distance such that the feed is started when the sewing needle is elevated to a predetermined position, and feed is terminated when the needle is moved downwardly to a predetermined position. Further, the feeding is performed by a predetermined amount or distance provisionally set. However, in spite of the feed with respect to the all needles under the even condition with each other, actual seam size and tension of the stitched thread may be different from one another due to difference in characteristics of the threads and difference in tensions applied to the threads. Furthermore, even if constant seam and tension are provided in all stitches, users may have mistaking visual impression that the seams and tensions are different from each of the threads due to the difference in diameters and colors of the threads. Such differences cannot be reduced by the even adjustment of the fabric feed timing and feed amount with respect to all stitches.